If you are trying to iChat with a PC user over an AirPort or AirPort Extreme connection, and it fails with a "user failed to respond" message, here's a possible fix:

Open up AirPort Admin Utility

Select your AirPort Base Station and click Configure.

Click on Network.

Make sure you UNcheck Distribute IP Addresses.

Update the configuration, and voila -- it worked for me.

[robg adds: We've had other tips on this subject, but I chose to run this one separately since it dealt with chats with PC users.

Warning: This hint applies only if you use your Base Station solely for a local network; if you connect to the Internet via your Base Station, and it's set to distribute addresses -- the default configuration -- please don't try this one!]

This is a ridiculous hint. It'll turn your base station into a switch, rather than a NAT router. The first device on your network to request an IP address will get one from directly from your ISP. Since most accounts only include a single IP, no other devices will be able to get internet access.

The proper solution to connectivity problems is to configure your router do to the appropriate port forwarding, not to disable its most important features.

Another fix for 'user failed to respond' in iChat AV
Authored by: robg on May 31, '06 12:03:46PM

I've added a clarifying warning -- this hint CAN work, and not break anything, if you use your Base Station as I do (and as does the original poster). My Base Station is used for wireless within my network, but the internet connection comes into a cable modem. The cable modem feeds a router, and the devices (including the Base Station) come off the router.

I apologize for not including this note in the original hint; I've used my Base Station this way for so long that I forgot it's far from the normal usage.

As someone who has benefited from this precise hint, I must disagree with the previous replyers.

Having iChat work with AirPort Express depends a lot on the type of network you are using. For those people, who have only one IP address assigned to the them by the ISP, this hint will most likely fail. (At least with multiple computers in the wireless network.)

However, my ISP allows up to five dynamical IP addresses. Using AirPort Express to take one of these and to build a NAT network around it leads to problems. It's better to allow any DHCP requests for an IP go up to the Internet Service Provider.

So there. This hint does not work for some people, but it will help others.